Without limiting the scope of the invention, its background is described in connection with resonant tunneling devices.
Integrated circuits have become the technology of choice for performing logic functions. The downscaling of minimum device geometries has provided for increases in the functional density and performance of integrated circuits. In electronic devices having nanometer dimensions, the wave behavior of electrons, namely interference and resonance, can be utilized to obtain useful electronic behavior for switching and logic.
A resonant tunneling diode (RTD) in its simplest embodiment consists of a sequence of five semiconductor layers. The outer two layers are the contact layers into which electrons enter and exit the semiconductor layer sequence. The interior three dissimilar semiconductor layers differ in their energy band gaps in the sequence wide/narrow/wide band gap with layer thicknesses comparable to the electron Bloch wavelength (typically less than 10 nm). This sequence of layers produces an energy profile through which electrons must travel and which consists of two energy barriers separated by a narrow region referred to as a quantum well.
Classically, an electron with energy, called the Fermi energy, approaching the first energy barrier with an energy below the barrier energy is reflected, analogous to a baseball rebounding off a concrete wall or to an electromagnetic wave at the end of an open-circuited transmission line. Quantum mechanics, however, allows that as the physical dimensions of the barrier decrease toward the wavelength of the particle, there is an increasing probability that the particle will be transmitted instead of reflected. Thus under certain conditions an electron can pass through the barrier even with energy below the barrier potential. This classically-forbidden phenomenon is called tunneling.
If the quantum well width is selected to be approximately equal to some integer or half-integer multiple of the electron wavelength, a standing wave can be built up by constructive interference analogous to the standing waves in a microwave cavity. Electrons at these wavelengths couple into and out of the quantum well more readily than others.
The electron energy, E, and its wavelength, I, are inversely related by the equation, E=h.sup.2 /2 ml.sup.2, where h is Planck's constant and m is the effective electron mass. Since the electron's energy can be controlled by adjusting the bias across the structure, the transmission (or current flow) through the double-barrier depends sensitively on the applied voltage. One can think of the double-barrier structure as an energy bandpass filter which transmits for certain applied biases and reflects the electron for other applied biases. The electron is said to be in resonance when the incoming electron energy matches the resonant transmission energy of the quantum-well structure.
In the RTD, the current increases monotonically with applied voltage until the average incoming electron energy is approximately equal to the resonance energy and the electron tunnels efficiently through the double-barrier structure. At slightly higher energy (applied bias) the electron no longer couples into the well efficiently and the transmission (current) is reduced. At still higher applied voltages, the electron's energy is sufficient for it to get over the barriers giving rise to an increasing current with bias. Thus the current-voltage characteristic of the resonant tunneling diode is N-shaped. It is this characteristic which is utilized to advantage in resonant tunneling electron devices.